Tuesday, 29 January 2013

Woodfired Cooking 101.1

Wood fired ovens have had a great surge in popularity in recent years and the most common questions we are asked both in cooking classes and in the invariably big boy’s huddle that forms in the courtyard around the oven during lunches is- How they work? And how to use them?
The oven here at Sunnybrae is a masonry bread oven designed and built by Alan Scott in 2003 [photo journal here]    that is a little different to some other wood fired ovens in that the construction,  unique to Alan’s designs are built in layers for very good heat insulation to enable stable cooking for a long time once the oven is fired up. There are essentially two different ways to use a masonry oven firstly with a fire inside and the front door open for Pizza or roasting and such and also with no fire, just residual heat built up after it has been fired.  Alan’s designs follow ancient traditions but also have very modern thermodynamic features that ensure stable temperatures are held that can be accurately read and interpreted.
These ovens work on slightly different principles to normal gas and electric ovens in that they use radiant heat to cook. The mass of the superheated oven radiates infra red heat evenly from all sides that penetrate the goodies that you are cooking as opposed to conventional ovens that heat the air in the oven to transfer the heat by induction to the ingredients to be cooked. Outdoor heaters and hot water circulating radiators are also infra red.Its a different heat.
You can use any dry timber for fuel to heat up the oven but if you are cooking with the fire inside the choice of wood is very important as it will impart flavour from the smoke.  For baking bread the oven is empty so any dry timber can  go in to heat it up. If you do not have a good supply of free or inexpensive timber you may need to consider that as an oven like this takes about a full wheelbarrow of wood to get it to baking temperature.  For roasting with a fire in, it takes about half that amount to get it up to minimum heat.  The indigenous species boundary timber plantations we put in here at Sunnybrae about 30 years ago are mature and now supply  most of the fuel for this oven. Every year we harvest any dead wood or fallen branches and this takes care of our needs for the oven and also some of the heating in the cottage.  The local mill at Birregurra also has off cuts that are very useful.
The unique principle of an Alan Scott oven is in the layering of its construction. The inner layer is masonry firebricks, the layer below that is a floating concrete slab, the layer below that is a vermiculite foam and the final layer is air. Old historic ovens sit on a base of sand or rocks or bricks. In the past it was thought that the more mass the oven sits on the more insulation it would have. But modern thermodynamic principles display that heat would rather flow into sand or rock or any dense mass rather than into air. Its the same principle as double glazed windows. Alan’s designs have a temperature probe in the centre of each layer, six in all, both at the top and at the bottom. So at any time you can tell the temperature of the chamber and how much heat is stored in the other layers.
So if the temperature of the bricks inside is say 220C at the top and at the bottom the temperature is right for baking bread but if the second layer is say 100C then it will cool too fast to cook the bread.
If the inner layer is 220C and the second layer is 250C and the third layer is 280C then the oven will be able to cook batch after batch as the outer layers send their heat into the centre rather than into the air.
Once you have been baking with a masonry oven you quickly learn how fast it heats and how slow it will drop temperature. Its a metaphysical bond that grows between you and the oven as you get to use it more and more.
  This is an introduction to a user’s guide to an Alan Scott Oven  part of a larger user’s guide to Sunnybrae infrastructure that I will pass on to the new owners.
Alan’s son continues the legacy here  http://ovencrafters.net/  



Anonymous said...

So the difference between the Scott oven and the Scotch oven is the air layer which allows the heat trapped in the thermal mass to transfer more rapidly. In the Scott oven is there an air intake to the air layer to facilitate the heat transfer. What I understand is that both use radiant heat but the Scott oven is engineered a little better. There must be an equation somewhere which says that a given cubic mass heated to a given temperature will sustain long enough to cook a certain no of kilos of dough. My question is do you use the pyrometric probes or do you light the oven and use your knowledge of it to know what you can cook within it at different times in its heat cycle, and which wood gives which heat. Gas and Elecimitricity can only give heat in a measure of BTU per Kh or Lph,different wood species will deliver a wide range of heat depending on dryness, density, stoke size and draft. Joisy Boy.

Sunnybrae and all who sail in her said...

Not quite... if you can imagine the layers that go right arround the oven with brick on the inner layers followed by a cement layer followed by the vermiculite layer with the last layer as air rather than rock or sand that the old ovens had. The outer layer of vermiculite when hotter than the center would rather conduct its heat into the cement layer and the brick layer rather than out into the air layer. I judge the amount of wood by its weight in the wheelbarrow.
You need more of a lighter less dense timber than say redgum. If I use the mill offcuts I fill the whole oven up but never use pine or related species for cooking or smoking with the fire in the oven. For bread or slow cooking there is no fire at all inside the oven.